(1) Field of the Invention
This invention generally relates to the field of fluid flow measurements and more specifically to a turbulence velocimetry technique by obtaining the Doppler spread of an acoustic signal only due to turbulent or fluctuating motion of scatterers in a fluid such as water.
(2) Statement of the Prior Art
It is becoming increasingly essential to know about fluid flow for ocean waters as it affects movements of ships therethrough. Furthermore, detection of the ships and the like, while at sea, is also important. Additionally, limiting the spreading of an oil spill in a body of water and the distribution of nutrients in the ocean affecting its ecological balance are also important. All these problems involve the study of turbulence fluid flow in the ocean. Thus it is desirable to estimate turbulence in the ocean and its impact under different conditions.
Doppler acoustic techniques are becoming increasingly important in fluid flow measurements. Estimation is typically made of the component of motion of the acoustic scatterers' parallel to the acoustic axis by locating the mean Doppler spectrum. This Doppler shift must be estimated within the zone of the Doppler spread or variance of this shift. The mean component of flow, which is defined as the spatial average over a scattering volume, in the direction of the acoustic axis produces a Doppler shift in the projected frequency from f.sub.O to f.sub.O +.DELTA.f.sub.O. There is also an accompanying Doppler spread, (.DELTA.f) of the spectrum due to the mean vector velocity component of the scatterer motion in the direction perpendicular to the acoustic axis and due to the turbulent or fluctuating component of flow of the scatterers within the scattering volume. Thus information on the vector components of fluid flow averaged over the scattering volume and information on the fluctuating component of flow within the scattering volume are contained in the Doppler spread spectrum. It is thus desired to infer this fluid flow information from estimations made of the Doppler spectrum.
Doppler spread, .DELTA.f, is typically the result of two components of the fluid flow of the scatterers: (1) the mean vector velocity component in the direction perpendicular to the acoustic axis and (2) the turbulent or fluctuating component of fluid flow of the motion of the scatterers within the scattering volume. In general, it is impossible to distinguish between the effects of (1) and (2) which contribute to the Doppler spread. Thus both of these components contribute to the uncertainty of a mean Doppler estimate. In addition, it is not possible in general to use the Doppler spread effects to estimate either the transverse flow or the turbulent flow without a bias effect of one on the other. It is thus desirable to eliminate one of the two components (i.e., transverse component of the scatterers' motion) in order to get a realistic measurement of the other (i.e., the component due to turbulent motion of the scatterers).